I definitely concur with the Percy Jackson recommendation--it definitely has been a hit with a wide variety of readers, and the spin off interest in mythology opens up a whole new list of reading possibilities.
If shorter books are appealing right now, you might leave a copy of the first Deltorra Quest book around the house. That has completely ensnared my son and I recently saw it on a middle school teacher's recommended reads list (which leads me to believe that short does not in this case equal simplistic).
Also (but not so short): Gregor the Overlander, The Mysterious Benedict Society, The Warriors, Eragon, Inkheart, Hatchet...
My DD10, who is also an obsessive reader, went through something like what you describe last summer. A couple of thoughts:
Audiobooks (especially first in a series books for the car or for bedtime). Don't try Percy Jackson this way--not an audio interpretation that is on par with the writing (imho). First books might include: Gregor the Overlander, Into the Wild (warriors), Hatchet, etc. If the first book appeals, there is likely to be interest in checking out the subsequent books.
Sprinkling books around the house/car: don't recommend, just leave books where he might happen to land when there is nothing else to do. Some books DD picked up, some she ignored. You might especially consider something with a lot of short stories in it. My DD has been reading the complete Brothers Grimm in fits and starts since early July. She probably wouldn't have picked it up if I hadn't left it in the backseat of the car, but it has become one of her favorite books to take along with us when she knows she's going to be sitting around and waiting someplace.
Re-reading is not a bad thing. Even though watching DD read a Harry Potter or Percy Jackson book for the 8 millionth time sometimes makes me grit my teeth, I think that re-reading is actually a positive thing. First read(s) of a book are pretty plot driven--we want to know what's going to happen. Subsequent readings are when we pick up all of the nuances of the character, language, setting....I hear DD interpret and discuss other books through her complete understanding of her "anchor" books. She knows these favorites so well now that she says things like, "it's like in "x" when the author makes you think...." or "it's sort of like how the character in "x" seems like "y", but the author drops little hints for you...".
Finally....books that seem "below" in ability are not really a bad thing either (imho). Books can/should sometimes be just for the pure escape of it all. Voracious readers will go through series like Captain Underpants very, very quickly (and sometimes pick up some GREAT vocabulary) and with a lot of enjoyment--it's the equivalent of the "beach read". Not much in the way of growth, but the kind of fun and harmless escape we all enjoy. DD sat down and read the first five books in a weekend, and ALWAYS grabs at least one book of comic strips from library (favorites: Calvin and Hobbes, Foxtrot, Peanuts).
Phew! Long post. HTH!
